In a mass spectrometer, ions separated in accordance with their mass-to-charge ratio m/z in a mass separator are detected in an ion detector. In general, in an ion detector, a signal proportional to the number of received ions is read out. In particular, in a quantitative analysis, it is important that the range of the amount of detectable ions, i.e. the dynamic range, is wide. Main restriction factors of the dynamic range are the upper limit of the amount of ions to be mass analyzed and the upper and lower limits of the amount of ions that the ion detector itself can detect.
For example, consider an ion trap time-of-flight mass spectrometer (IT-TOFMS) in which a three-dimensional quadrupole ion trap and a time-of-flight mass spectrometer are combined. A three-dimensional quadrupole ion trap has a relatively low upper limit of the amount of ions that can be stored. In addition, even when the amount of ions is lower than the upper limit, if the amount of ions stored in the ion trap is large, a deterioration of performance, such as the mass resolving power, disadvantageously occurs due to the effect of the interaction among the ions called a space-charge effect. On the other hand, a linear ion trap has a high upper limit of the amount of ions that can be stored compared to three-dimensional quadrupole ion traps. Hence, a use of a linear ion trap in an IT-TOFMS allows a mass analysis of a larger amount of ions, which is advantageous in expanding the dynamic range. After ion optical properties on the ion supply side are improved as just described, what is important in expanding the dynamic range is an improvement of the dynamic range of the ion detector itself.
Examples of ion detectors widely used in a mass spectrometer are as follows: an ion detector using a secondary electron multiplier (refer to Patent Document 1 and other documents); an ion detector using the combination of a conversion dynode and a secondary electron multiplier (refer to Patent Document 2 and other documents); an ion detector using the combination of a conversion dynode, fluorescence substance and a photoelectron multiplier. For example, as disclosed in Patent Document 1 and other documents, in a general secondary electron multiplier, a high voltage provided from a direct-current power supply is resistively divided and applied to multi-stage dynodes for multiplying electrons. The multiplication factor, i.e. the gain of the detector, is changed by controlling the voltage provided from the direct-current power supply.
In a detector using an electron multiplication technology such as a secondary electron multiplier, a photoelectron multiplier, or other unit, the multiplication factor decreases when the input is too much (in particular, when the amount of entering ions is too much), when the voltage applied to dynodes is insufficient, or in other case. This disadvantageously results in saturation of an output signal which is read out from the anode provided in the final stage (which is sometimes called a collector). As methods for resolving such a problem, a boosting method and a dynode readout method are conventionally known.
In the boosting method, the power feeding is not performed by a resistive division but independently performed to each of the dynodes where secondary electrons are multiplied or to one or more dynodes in the posterior portion so that those applied voltages can be adjusted at will. In the dynode readout method, a signal is read out not only from an anode but also from one or more dynodes where electrons are multiplied,
However, even with such conventional methods as just described, it is difficult to sufficiently improve the dynamic range. For example, in a TOFMS, a large number of ions continuously enter the ion detector in a very short period of time. In such a case, even if a power is supplied independently to each of the dynodes as in the boosting method, the power feeding amount may transiently run short or a space-charge effect may occur by the electrons inside the secondary electron multiplier, which may lead to a temporary decrease in gain or a rounding of the output waveform. Even in the case where a sufficient power is fed to the dynodes and the space-charge effect of the electrons in the secondary electron multiplier is negligible, in a TOFMS in which a high-speed waveform must be detected, it is necessary to broaden the input band of the amplifier of the detection signal and simultaneously set a high sampling frequency. Consequently, the noise level due to the thermal noise is not negligible, which becomes a restriction factor of the dynamic range.
When a signal is read out from each of the intermediate dynodes or from a specific dynode in a secondary electron multiplier as in the dynode readout method, even in the ease where a decrease in gain or a rounding of waveform occurs in the signal of the anode which is placed in the last stage, the decrease in gain and the rounding of waveform in the intermediate dynodes are relatively small. Therefore, a use of signals of the intermediate dynodes can prevent saturation of the output even when the input is too much.
However, even if there is no longer an excessive input, it is not possible to ensure a sufficient gain for a low-level input immediately after an excessive input, since the secondary electron multiplier requires a certain amount of time to recover from a decrease in gain and a rounding of waveform at each of the dynodes in the posterior portion and the anode. This constitutes a factor of restricting the dynamic range and deteriorating the quantitative capability. Further, in the dynode readout method, it is necessary to process a plurality of signals provided from the secondary electron multiplier. Accordingly, due to the arithmetic computation, the cost of the signal processing unit may be increased. Further, the processing speed may be restricted due to the large amount of computation.